Dear Rooters,
I would like to report a bug in fitting a TGraphError:
If I fit a TGraphError with a polynomial function, with the parameter 0 fixed
to 0. ,
then results are not in agreement with initials values. I mean, p0 is well
fixed, but not equal to 0.
If function f1 is described by
TF1 *f1 = new TF1("f1","[0]+[1]*x",0.,5.);
then it works.
I'm working with 2.23.12 version on UNIX system.
Sorry if this problem has been already reported.
Stephanie.
void test()
{
Float_t x[4]={1.,2.5,3.5,4.75};
Float_t ex[4]={0.01,0.01,0.01,0.01};
Float_t y[4]={0.,1.,2.,3.};
Float_t ey[4]={0.01,0.01,0.01,0.01};
TGraphErrors* g1 =new TGraphErrors(4,x,y,ex,ey);
TF1 *f1 = new TF1("f1","pol1",0.,5.);
f1->SetParameter(0,0.);
f1->SetParLimits(0,1,1);
TCanvas* c1;
c1 = new TCanvas("c1","c1",100,10,800,500);
c1->cd();gStyle->SetOptFit(111);
g1->Draw("AP");
g1->Fit("f1","B");
}
Print on screen:
FCN=2442.08 FROM MIGRAD STATUS=CONVERGED 18 CALLS 19 TOTAL
EDM=4.66971e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 -2.75862e-01 fixed
2 p1 6.47669e-01 1.72543e-03 4.16384e-05 -1.77117e-01
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